Application of a low acetate-producing strain of Tetragenococcus halophilus to soy sauce fermentation

In soy sauce brewing, the halophilic lactic acid bacterium, Tetragenococcus halophilus is used as a fermentation starter and contributes to the taste and aroma of soy sauce, mainly by producing lactate. By lowering the pH of the soy sauce mash, lactate serves as a suitable growth environment for the halotolerant yeast Zygosaccharomyces rouxii. Acetate, which is produced by T. halophilus via the citrate metabolic pathway, is a critical growth inhibitory factor for Z. rouxii. Therefore, a T. halophilus strain that lacks acetate production could be an ideal fermentation starter to enhance ethanol production. In this study, we obtained a derivative of T. halophilus containing an insertion sequence in citC, which is an essential gene for citrate metabolism, and validated its performance as a soy sauce fermentation starter. The derivative neither metabolized citrate nor produced excessive acetate in soy sauce mash, resulting in vigorous alcohol fermentation by Z. rouxii. This study provides insights into the application of a low acetate-producing strain of T. halophilus as a starter to produce soy sauce with high alcohol content and low sour aroma.